1. Field of the Invention
The present invention relates to a catadioptric projection optical system suitable for applications to projection optical systems for 1:1 or demagnifying projection in projection exposure apparatus such as steppers used in fabricating, for example, semiconductor devices or liquid crystal display devices, etc., by photolithography process. More particularly, the invention relates to a catadiontric projection optical system of a magnification of xc2xc to ⅕ with a resolution of submicron order in the ultraviolet wavelength region, using a reflecting system as an element in the optical system.
2. Related Background Art
In fabricating semiconductor devices or liquid crystal display devices, etc. by photolithography process, the projection exposure apparatus is used for demagnifying through a projection optical system a pattern image on a reticle (or photomask, etc.) for example at a ratio of about xc2xc to ⅕ to effect exposure of the image on a wafer (or glass plate, etc.) coated with a photoresist or the like.
The projection exposure apparatus with a catadioptric projection optical system is disclosed, for example, in Japanese Laid-open Patent Application No. 2-66510, Japanese Laid-open Patent Application No. 3-282527, U.S. Pat. No. 5,089,913, Japanese Laid-open Patent Application No. 5-72478, or U.S. Pat. Nos. 5,052,763, 4,779,966, 4,65,77, 4,701,035.
An object of the present invention is to provide an exposure apparatus having a catadioptric projection optical system which can use a beam splitting optical system smaller than the conventional polarizing beam splitter and which is excellent in image-forming performance, permitting a sufficiently long optical path of from the concave, reflective mirror to the image plane. Therefore, the catadioptric projection optical system has a space permitting an aperture stop to be set therein, based on a size reduction of the beam splitting optical system such as a polarizing beam splitter. The catadioptric projection optical system can be applied to the projection exposure apparatus of the scanning exposure method, based on use of a compact beam splitting optical system. Besides the projection exposure apparatus of the one-shot exposure method, the catadioptric projection optical system can be also applier to recent apparatus employing a scanning exposure method such as the slit scan method or the step-and-scan method, etc. for effecting exposure while relatively scanning the reticle and the wafer to the projection optical system.
To achieve the above object, as shown in FIG. 1, an exposure apparatus of the present invention comprises at least a wafer stage 3 allowing a photosensitive substrate W to be held on a main surface thereof, an illumination optical system 1 for emitting exposure light of a predetermined wavelength and transferring a predetermined pattern of a mask (reticle R) onto the substrate W, a catadioptric projection optical system 5 provided between a first surface P1 on which the mask R is disposed and a second surface P2 to which a surface of the substrate W is corresponded, for projecting an image of the pattern of the mask R onto the substrate W. The illumination optical system 1 includes an alignment optical system 110 for adjusting a relative positions between the mask R and the wafer W, and the mask R is disposed on a reticle stage 2 which is movable in parallel with respect to the main surface of the wafer stage 3. The catadiodtric projection optical system has a space permitting an aperture stop 6 to be set therein. The sensitive substrate W comprises a wafer 8 such as a silicon wafer or a glass plate, etc., and a photosensitive material 7 such as a photoresist or the like coating a surface of the wafer 8.
In particular, as shown in FIGS. 2, 17, and 31, the catadioptric projection optical system comprises a first image-forming optical system (G1(f1),G2(f2)) for forming an intermediate image 11 of the pattern of the mask R, and a second image-forming optical system (G3(f3)) for forming an image of the intermediate image 11 on the substrate W. The first image-forming optical system has a first group G1(f1) with a positive refractive power, comprising a refractive lens component, for converging a light beam from the pattern of the mask R, a second group G2(f2) with a positive a refractive power, comprising a concave, reflective mirror M2 for reflecting a light beam from the first group G1(f1), for forming the intermediate image 11 of the pattern of the mask R, and a beam splitting optical system 10PBS (including 10A, 10B, and 10C) or an optical path changing mirror 12 as a beam splitting optical system for changing a traveling direction of one of a light beam from the first group G1(f1) and a reflected light from the concave, reflective mirror M2, and thereby a part of the light beam converged by the second group G2(f2) is guided to the second image-forming optical system G3(f3). The parameter f1 means as a focus length of the first group G1 in the first image-forming optical system, the parameter f2 means as a focus length of the second group G2 in the first image-forming optical system, and the parameter f3 means as a focus length of a lens group G3 in the second image-forming optical system.
The catadioptric projection optical system in FIG. 2 is an optical system for projecting an image of a pattern of a first surface P1 onto a second surface P2, which has a first image-forming optical system (G1, G2) for forming an intermediate image 11 of the pattern of the first surface P1 and a second image-forming optical system (G3) for forming an image of the intermediate image 11 on the second surface P2.
The first image-forming optical system comprises a first group G1(f1) of a positive refractive power, comprising a refractive lens component, for converging a light beam from the pattern of the first surface P1, a prism type beam splitter 10PBS for separating a part of a light beam from the first group by a beam splitter surface 10PBSa arranged obliquely to the optical axis AX1 of the first group, and a second group G2(f2) with a positive refractive power, comprising a concave, reflective mirror M2 for reflecting the light beam separated by the prism type beam splitter 10PBS, for forming the intermediate image 11 of the pattern near the prism type beam splitter 10PBS, in which a part of the light beam converged by the second group G2(f2) is separated by the prism type beam splitter 10PBS to be guided to the second image-forming optical system G3(f3). The prism type beam splitter is disposed on the optical axis AX1 of the first group G1(f1) and provided between the concave, reflective mirror M2 and the second image-forming optical system.
In this case, it is desirable that the intermediate image 11 of the pattern be formed inside the prism type beam splitter 10PBS. Also, as shown in FIG. 2, it is desired that in order to prevent generation of flare due to repetitive reflections between the concave, reflective mirror M2 and the second surface P2, a polarizing b spit-per be used as the beam splitter 10PBS and a quarter wave plate 9 be placed between the polarizing beam splitter and the concave, reflective mirror M2. Further, it is desired that the optical system be telecentric at least on the image plane P2 side.
Next, the catadioptric projection optical system in FIG. 17 is an optical system for projecting an image of a pattern P10 on a first surface P1 onto a second surface P2 which has a first image-forming optical system (G1(f1), G2(f2)). for forming an intermediate image 11 of the pattern P10 of the first surface P1, and a second image-forming optical system (G3(f3)) forming an image of the intermediate image 11 on the second surface P2.
The first image-forming optical system comprises a first group G1(f1) of a positive refractive power, comprising a refractive lens component, for converging a light beam from the pattern P10 of the first surface P1, a partial mirror 12 for separating a part of the light beam from the first group by a first reflective surface 12a arranged obliquely to the optical axis AX1 of the first group, and a second group G2(f2) of a positive refractive power, comprising a concave, reflective mirror M2 for reflecting the light beam of which the part is separated by the partial mirror 12, for forming the intermediate image 11 of the pattern P10 near the partial mirror 12, in which a Dart of the light beam converged by the second group is guided to the second image-forming optical system G3(f3). The partial mirror 12 is positioned so as to avoid being disposed on the optical axis AX1 of the first group and provided between the first group and the second group. The partial mirror 12 further has a second reflective surface for guiding the reflected light beam from the concave, reflective mirror M2 to the second image-forming optical system, the second reflective surface 12b being opposite to the first reflective surface 12a.
In this case, because the light beam reflected by a second surface 12b of the partial mirror 12 is used, it is desired that an image-forming range be slit or arcuate. Namely, the catadioptric projection optical system in FIG. 17 is suitable for applications to the projection exposure apparatus of the scanning exposure method. In this case, because the use of the partial mirror 12 includes little influence of repetitive reflections, the quarter wave plate can be obviated.
In these arrangements, the following conditions should be preferably satisfied when individual Petzval sums of the first group G1(f1), the second group G2(f2), and the second image-forming optical system G3(f3) are P1, P2, P3, respectively.
p1+p3 greater than 0 xe2x80x83xe2x80x83(1) 
p2 less than 0 xe2x80x83xe2x80x83(2) 
|p1+p2+p3| less than 0.1 xe2x80x83xe2x80x83(3) 
Further, the following conditions should be preferably satisfied when a magnification of primary image formation of from the pattern on the first surface P1 to the intermediate image is xcex212, a magnification of secondary image formation of from the intermediate image to the image on the second surface P2 is xcex23, and a magnification of from the first surface to the second surface is xcex2.
0.1xe2x89xa6|xcex212|xe2x89xa60.5 xe2x80x83xe2x80x83(4) 
0.25xe2x89xa6|xcex23xe2x89xa62 xe2x80x83xe2x80x83(5) 
0.1xe2x89xa6|xcex2|xe2x89xa60.5 xe2x80x83xe2x80x83(6) 
The catadioptric projection optical system in FIG. 2 is suitably applicable to the projection exposure apparatus of the one-shot exposure method. In this case, because the prism type beam splitter 10PBS is used to separate the light beam coming from the concave, reflective mirror M2 from the light beam going to the concave, reflective mirror M2 and because the beam splitter 10PBS is located near the portion where the light beam from the concave, reflective mirror M2 is once converged to be focused, the prism type beam splitter 10PBS can be constructed in a reduced scale. In other words, in the catadioptric projection optical system, since an intermediate image 11 of the pattern of the first surface P1 is formed between the concave, reflective mirror M2 and the second image-forming optical system, the diameter of the light beam traveling from the concave, reflective mirror M2 to the beam splitter 10PBS will become small.
Also, because the image is once formed between the concave, reflective mirror M2 and the image plane P2, an aperture stop 6 can be placed in the second image-forming optical system G3(f3). Accordingly, a coherence factor ("sgr"value) can be readily controlled. With regard to this, because after the primary image formation, the secondary image formation is made by the second image-forming optical system G3(f3), the working distance between a fore end lens in the second image-forming optical system G3(f3) and the image plane P2 can be secured sufficiently long. In particular, because the projection exposure apparatus of the one-shot exposure method employs the beam splitter 10PBS located near the plane of primary image formation, the beam splitter 10PBS can be made as small as possible.
Next, because the catadioptric projection optical system in FIG. 17 uses the partial mirror 12, a best image region on the image plane P2 is slit or arcuate, thus being suitable for applications to the projection exposure apparatus of the scanning exposure method. In this case, because the image is once formed near the partial mirror 12, the partial mirror 12 may be small in size and characteristics of a reflective film of the partial mirror 12 are stable.
Also, the optical path can be separated simply by providing the partial mirror 12 with a small angle of view. Namely, because a large angle of view is unnecessary for separation of the optical path, a sufficient margin is left in the image-forming performance. With regard to this, ordinary catadioptric projection optical systems need a maximum angle of view of about 20xc2x0 or more for separation of the optical path, while an angle of view of the light beam entering the partial mirror 12 is about 10xc2x0, which is easy in aberration correction.
A so-called ring field optical system is known as a projection optical system for the scanning exposure method, and the ring field optical system is constructed to illuminate only an off-axis annular portion. It is, however, difficult for the ring field optical system to have a large numerical aperture, because it uses an off-axis beam. Further, because optical members in that system are not symmetric with respect to the optical axis, processing, inspection, and adjustment of the optical members are difficult, and accuracy control or accuracy maintenance is also difficult. In contrast with it, because the angle of view is not large in the present invention, the optical system is constructed in a structure with less eclipse of beam.
Since the first image-forming optical system (G1(f1), G2(f2)) and the second image-forming optical system G3(f3) are constructed independently of each other, the optical system is easy in processing, inspection, and adjustment of optical members, is easy in accuracy control and accuracy maintenance, and has excellent image-forming characteristics to realize a large numerical aperture.
Next, in the catadioptric projection optical system shown in FIG. 2 or 17, a Petzval sum of the entire optical system first needs to be set as close to 0, in order to further improve the performance of optical system. Therefore, conditions of equations (1) to (3) should be preferably satisfied.
Satisfying the conditions of equations (1) to (3) prevents curvature of the image plane in the optical performance, which thus makes flatness of the image plane excellent. Above the upper limit of the condition of equation (3) (or if p1+P2+P3xe2x89xa70.1), the image plane is curved as concave to the object plane; below the lower limit of the condition of equation (3) (or if p1+p2p3xe2x89xa60.1), the image plane is curved as convex to the object, thereby considerably degrading the image-forming performance.
When the conditions of equations (4) to (6) are satisfied as to the magnification xcex212 of primary image formation, the magnification xcex23 of secondary image formation, and the magnification xcex2 of overall image formation, the optical system can be constructed without difficulties. Below the lower limit of each condition of equation (4) to (6), the demagnifying ratio becomes excessive, which makes wide-range exposure difficult. Above the upper limit, the demagnifying ratio becomes closer to magnifying ratios, which is against the original purpose of use for reduction projection in applications to the projection exposure apparatus.
In this case, because the condition of equation (4) is satisfied, the most Dart of the demagnifying ratio of the overall optical system relies on the first image-forming optical system. Accordingly, the beam splitter 10PBS or the partial mirror 12 can be constructed in a small scale in particular. If the position of the beam splitter 10PBS in FIG. 2 or the partial mirror 12 in FIG. 6 as beam splitting means is made nearly coincident with the entrance pupil and the exit pupil of optical system, a shield portion on the pupil does not change against a change of object height, and therefore, no change of image-forming performance appears across the entire image plane.
Also, it is desired that such an optical system for exposure be telecentric at least on the image plane side in order to suppress a change of magnification against variations in the direction of the optical axis, of the image plane where the wafer or the like is located.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art form this detailed description.